The present disclosure relates to the physical components by which operational control functions are implemented in a respirator to control gas flow and pressure delivered to the patient. As respirator technology has developed, great efforts have been made to enhance the limited phsyical behavior of available valves and other mechanical devices used for controlling gas flow and pressure. These enhancements have resulted in the production of ventilators of increasing mechanical and operational complexity, despite the fact that the available respirator still preform only relatively basic control functions. To accommodate the limitations of key physical components, such as valves, "specialty" respirators or ventilators which can perform only a portion of the total therapeutic range of treatment have been needed to carry out more advanced therapeutic modes, such as High Frequency Ventilation.
For a respirator to perform over the full range of needed functions for all patients of any age or condition, the control mechanisms for the gas delivery system must be capable of the most demanding mechanical tasks, namely High Frequency Ventilation (HFV) and therapy for tiny, premature infants. Mechanical reliability, functional stability in conventional settings for Continuous Positive Airway Pressure (CPAP) and Intermittent Mandatory Ventilation (IMV) and ease of manufacture are typically favored by designing larger dimension components. However, the larger mass and compression volume encountered in the use of larger components prevents them from operating effectively at higher ventilating frequencies and with smaller patients.
Using smaller dimension components to allow high frequency response increases mechanical instability. This instability, in turn, must be manageably by reliable means to allow use with the conventional therapy modes. Finally, manufaturing technology requirements should, if possible, be reduced by relying more upon electronic means for the elaborate control processing such a multi-function ventilator requires. Mechanical/pneumatic control devices should therefore be kept small in size, simple in operation, readily maintainable, and inexpensive to manufacture. Operation and stability should be primarily managed by means of computer controls. This increases functional capability of the system and reduces its manufacturing costs.
The present invention relates to a pulmonary ventilation assistance system, designed in the preferred embodiment specifically, but not limited to this application, for use with very low birth weight premature infants. The following important limitations of current respirator technology are addressed by the present invention:
(1) mechanical limitations to the prevention of undesirable pressure waves in the patient circuit,
(2) limited ability to design optimal pressure and flow waveforms;
(3) less than desirable variety of modalities of therapy; and
(4) limited provision for on-line measurement and recording of the mechanical properties of the patient's breathing system.
Delivery of breathing assistance to very small infants requires the ability to develop precisly-controlled gas flow delivery for relatively brief time intervals. Excessive pressure levels, and the occurrence of spikes in pressure during any part of the breathing cycle, must be prevented to limit mechanical injury to the airway tissues. Since current ventilator technology uses only two reference levels, with timed switching between them to provide the "cycling" involved with breathing assistance, a surge of pressure is typically produced in the breathing circuit with each switching from the lower-pressure reference level to the higher-pressure reference level at the exhale relief valve.
Typical respirator system use a constant flow of breathable gas into the patient circuit, with the exhale valve controlling the level of pressure achieved during cycling between low and high levels. Without precise flow-rate-vs-time control during each cycle, the operator is severely limited in the ability to provide an optimum mechanical transfer function between the machine and the patient's lungs.
The capability to produce breathing cycle frequencies, from zero (for Continuous Positive Airway Pressure) to as high as 20 Hz (for High Frequency Ventilation), is needed for optimum function. No current technology allows use of this full range in one instrument. Further, research with experimental techniques of breathing assistance, using combinations of different types of ventilation assistance devices, has shown the value of combinations of modalities. One such method superimposes a High Frequency Ventilation (HFV) waveform over simultaneous Intermittent Mandatory Ventilation (IMV) cycling. Another typical combination in such research is alternating periods of High Frequency Ventilation (HFV) and periods of either conventional Intermittent Mandatory Ventilation (IMV) or Continuous Positive Airway Pressure (CPAP). Further research will certainly reveal additional functions which may have merit with specific conditions. Again, no single instrument presently allows such combinations and complex cycling waveforms or the facility to conveniently provide newly designed ones in the future.
While several studies involving intermittent pulmonary function analysis have been reported, no ventilatory support device is available which measures such basic pulmonary function parameters as tidal volume, compliance and airway resistance in newborns continuously in the course of delivering ventilatory assistance. Research systems typically interpose gas flow sensing devices directly into the endotracheal tube attachment portion of the breathing circuit, allowing precise measurement of gas flow and airway pressure. Such a setup increases the "dead space" volume of the patient's airway and may result in significant re-breathing of expired gas. Additionally, these measuring devices are inherently awkward when mounted in this location, can be dangerous to the patient (due to the high temperature of the flow sensor), and are subject to contamination from patient secretions. While measurements thus derived have been useful in defining disease patterns in general, individual patient care has not been routinely monitored, nor therapy guided by these intermittent measurements because of the above technical difficulties and the requirement of analyzing the data by manual methods. Research indicates optimization of ventilator therapy could be greatly facilitated by such data being available automatically, at any time, on every patient. A system proposing to do this must not interfere with on-going therapy or introduce additional risks to the patient. The present invention fills this need by means made possible by the reduced patient circuit volume and by computer control and measurement of flow and pressure in that circuit.
The prior art ventilator control systems may be grouped according to the interaction between electronic, mechanical and pneumatic components with regard to producing patterns of pressure and flow. Totally pneumatic and fluidic/pneumatic systems require no electrical power for operation. Electric motor driven, adjustable linkage piston or valve systems require electrical power and pressurized gas supply but control rate of function by the operator adjusting the motor speed. Electric solenoid valve systems use electrical power to time cycle intervals and switch pneumatic lines, but leave control of pressure and gas flow waveforms to mechanical/pneumatic devices. An additional group of respirator designs utilizes industrial proportional electric/pneumatic interface valves to directly govern pressure and gas flow waveform shapes. The present systems should be classified in this latter group.
Totally fluidic and pneumatic/fluidic systems offer simplicity of supply; using only compressed oxygen and air for both breathing gas and control functions. However, even when only basic ventilator therapy functions are offered, totally pneumatic systems must become very complex to perform acceptably. An example is illustrated by the profusion of separate devices and fittings involved in the Bird Ventilator designs U.S. Pat. Nos. 4,197,843 and 4,592,349).
Additionally, since pressure and gas flow waveform generation is primarily a mechanical function, the fixed physical properties (mass of moving parts, friction, compressible volumes, fixed restrictor orifices, etc.) of the components in fluidic based systems tend to limit their adjustable range. Currently available systems designed for operational stability in CPAP and IMV modes do not have adequate adjustable range to provide High Frequency Ventilation unless an additional set of actuation components is installed.
Finally, temperature stability of gas supplies is critical to stable operation with purely pneumatic or fluidic designs. Variations in input gas temperature will change the function of timing devices and may alter air/oxygen mixture, breathing gas flow rate and pressure levels. Due to the complexity of construction, repairs and adjustments tend to be difficult to perform.
Electric motor driven piston or valve systems utilize the sinusoidal waveform generation property of reciprocating piston pumps. The sturdiness of the relatively larger-scale components lends greater stability to cycle timing and output performance. However, the fixed properties of the components restrict their operational range. Expansion to include the full range of modern therapy modes, including CPAP, IMV and HFV, require addition of many fluidic and pneumatic components. On the other hand, purely fluidic and fluidic/pneumatic systems capable of duplicating the piston's sinusoidal function, plus CPAP, IMV and HFV, would be very complex mechanically.
Electric solenoid valve systems enhance the totally pneumatic and piston systems with electrical timing control. Pneumatic flow and pressure waveform generation remain as mechanical functions, again limited in range by the physical properties of the components. However, complex timing and electronic feedback control becomes possible. Extension of function into some of the newer therapeutic modes is also relatively easier than with the piston type designs.
Far greater control complexity may be obtained in an electronically controlled system for a tiny fraction of the manufacturing cost of achieving such capability with purely pneumatic or motor-driven mechanical systems. Maintenance of electronic circuitry is far simple and less costly than for pneumatic components performing similar functions. With the addition of electronic pressure transducers, alarm responses can be added and computation and display of such data as mean airway pressure becomes feasible.
Solenoid valve operated ventilators currently provide the major share of medical therapy and are well accepted by medical therapists. Due to fixed limitations of components, these are designed to be specialized to certain ranges of patient size. The High Frequency Ventilator designs by Ellestad, et al. U.S Pat. No. 4,351,329), Miodownik U.S. Pat. No. 4,450,838) and by Bunnell, et al. U.S. Pat. No. 4,481,944 and 4,538,604) also fit in this group and are likewise specialized to produce only HFV.
However, the growing interest and successful experience with High Frequency Ventilation therapy has encountered difficulties with clinical implementation. typical HFV therapy actually requires a "full-range" ventilator setup which will provide CPAP and IMV modes as well as HFV. Such combinations must presently be jury rigged by therapists. The manufacturers of conventional CPAP/IMV ventilators and of HFV units consider themselves marketing competitors and have not yet produced acceptable means of combining their individual products with those produced by competing companies. The design of "full-range" ventilators in an active area of product research and development.
Proportional electric/pneumatic interface valves (EP transducers), manufactured for industrial process control, are proposed in recent patents as means for generating the complex pressure and flow waveforms needed for optimal ventilator control. Boyarsky, et al. U.S Pat. No. 4,393,869) utilize an electrical to pneumatic transducer to pilot control a breathing gas supply regulator. This application allows a degree of proportional feedback control of function. Unfortunately, the success of this application is limited by the fact that available industrial EP transducers operate out of range (3-15 psi) of the pressure levels needed for ventilator controls (-0.1 to 2.0 psi). Specifically, the most critical precision range for the ventilator valve control system, is between -0.1 to 0.1 psi, especially for use with tiny premature infants and HFV. Additional pneumatic devices are needed to translate the higher control pressures from the EP transducer into the ventilator operational range. This two-stage approach results in mechanical degradation of response characteristics. Generation and feedback control of High Frequency Ventilation functions would be especially difficult.
DeVries et al. U.S. Pat. No. 4,527,557) proposes an electronically adjustable Venturi pressure generator system. The output pressure is governed by a variable restrictor valve in the exhaust port of the Venturi tube. A stepper motor, cam and cam follower are used to move the variable restrictor valve element according to electronic control signals. The preferred embodiment of this device is listed as a flow controller to systematically adjust the inspiratory flow waveform of a ventilator. In this application, the functional waveform of a gas flow regulator is pneumatically adjusted by the variable output pressure from the Venturi pressure generator.
While remarkably simpler and more capable of pressure and flow waveform control than the prior art, this valve design remains functionally limited by the physical properties of certain of its components. The frequency response needed for a full-range ventilator system is expensive and difficult, if not impossible, with available stepper motors in this application. While an appropriately designed Venturi reference pressure generator can, potentially, have the desired frequency response, the choice of stepper motor, cam and follower to adjust it is unfortunate. The mechanical inertia and friction of the motor armature, cam and follower, plus the need to step through each step from one position to another, combine to slow response. The limited number of steps in a segment of rotation of the motor also reduces resolution. Advances in stepper motor technology will undoubtedly improve function, possibly even enough to overcome some of these problems. The expense and complexity of such motors will, however, likely continue to be limiting factors.
The system described herein, while mechanically less complex than systems shown by the prior art, uinquely offers to the user a full range of operational control from Continuous Positive Airway Pressure (CPAP), through the full range of Intermittent Mandatory Ventilation (IMV), to the upper limits of High Frequency Ventilation (HFV). This is attained because the majority of system control comes from the operation of computer software. The electro-pneumatic interface of the system is an advancement over the prior art in being capable of covering this full range, including accurate high frequency response.